I thought I'd add my 2-cents. My Massdrop avatar photo since I joined happens to be me in my Koss esp-950s.
They were long my favourite headphones, but have been replaced in that top spot by Audeze LCD-3, but they're still a close 2nd.
I love the electrostatic sound, for midrange and high-end it's beautifully detailed, accurate and responsive. These won't be bass monsters by any means. Switching back and forth between the LCD-3 and the 950s when listening to anything with a lot of bass will leave them sounding a bit thin. But bass just isn't where they shine. I tend to use them for rock music, classical, jazz but not techno, EDM/hip-hop.
There's a sweet-spot around 1000Hz and up where there is a slight warmth that really makes acoustic music and female vocals, very warm and non-fatiguing as other headphones might be in the upper end.
I've had my 950s for over ten years and my time hasn't been without technical issues. I had them for years when the notorious "buzz issue" started up. The right side would get this faint intermittent buzzing sound that would be drowned out by music but still annoying. I sent them in to Koss and must praise the warranty here.
I started with a phone call for instructions and talked to a really nice lady at the Koss headquarters. The whole process of shipping from my home in Ontario, Canada to their headquarters in Milwaukee and back again repaired took about two weeks. To my surprise they not only fixed the buzz but they slapped on brand new earpads and even replaced the cables with new ones. The fact that this service exists, no questions asked, no proof of purchase, no wacky verification number requirement - for life, cannot be understated. Koss deserves much praise for their excellent support policy.
I still use them semi-regularly. They are my go-to for jazz, acoustic music. I turn to my Fostex/Massdrop TH-X00 purpleheart for anything that benefits from a more bass-forward presentation (such as electronica), so I feel like I've got the best of both worlds between the two headphones.
It's a shame the battery pack won't be included with this drop. I love the battery pack, but I admit I hardly use it anymore. Attaching a battery pack to the "energizer" is ridiculous fun just because it's so impractical.
The overall build quality and feel by today's standards might seem a bit "cheap". The earcups are plastic, but seem to be more solid than they feel because I've never broken them or had any issues with cracks. They're extremely light on your head, they almost feel like "nothing"... especially compared to my LCD-3s which are pretty heavy. The build quality and design are definitely out of the past, but know what you're getting into. They're vintage headphones, but good sound is eternal. And that lifetime warranty will keep them eternal - so it's easy to overlook the plasticiness of the shells around the earcups.
I guess it's worth noting you can get aftermarket earpads, I ordered them online and they're a bit thicker than Koss's stock earpads . There might be a slight bump in the mid-bass due to the earpads and their placement being slightly further from my head, but honestly I didn't notice much difference in sound quality.
I hope you enjoy the drop, good luck.